Problem: Which of these numbers is prime? ${15,\ 35,\ 37,\ 39,\ 94}$
A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 35 are 1, 5, 7, and 35. The factors of 37 are 1 and 37. The factors of 39 are 1, 3, 13, and 39. The factors of 94 are 1, 2, 47, and 94. Thus, 37 is a prime number.